Method and apparatus for cardiac shock therapy

ABSTRACT

An apparatus and method for delivering defibrillation shock therapy employing a multi-terminal pulse output circuit. In such a circuit, at least three electrode lead terminals are switchably connected to the positive and negative terminals of an energy storage capacitor. By serially switching selected electrode lead terminals to the capacitor terminals, a variety of shock pulse waveforms may be generated.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.09/754,099, filed on Jan. 4, 2001, now issued as U.S. Pat. No.6,668,193, the specification of which is incorporated herein byreference.

FIELD OF THE INVENTION

This invention pertains to apparatus and methods for treating cardiacarrhythmias. In particular, the invention relates to an apparatus andmethod for electrically terminating tachyarrhythmias.

BACKGROUND

Tachyarrhythmias are abnormal heart rhythms characterized by a rapidheart rate. Examples of tachyarrhythmias include supraventriculartachycardias such as sinus tachycardia, atrial tachycardia, and atrialfibrillation (AF), and ventricular tachyarrhythmias such as ventriculartachycardia (VT) and ventricular fibrillation (VF). Both ventriculartachycardia and ventricular fibrillation are hemodynamicallycompromising, and both can be life-threatening. Ventricularfibrillation, however, causes circulatory arrest within seconds and isthe most common cause of sudden cardiac death. Atrial fibrillation isnot immediately life threatening, but since atrial contraction is lost,the ventricles are not filled to capacity before systole which reducescardiac output. This may cause lightheadedness or fainting in someindividuals, as well as fatigue and shortness of breath, hindering theindividual from carrying out normal daily activities. If atrialfibrillation remains untreated for long periods of time, it can alsocause blood to clot in the left atrium, possibly forming emboli andplacing patients at risk for stroke.

Cardioversion (an electrical shock delivered to the heart synchronouslywith an intrinsic depolarization) and defibrillation (an electricalshock delivered without such synchronization) can be used to terminatemost tachyarrhythmias, including AF, VT, and VF. As used herein, theterm defibrillation should be taken to mean an electrical shockdelivered either synchronously or not in order to terminate afibrillation. In electrical defibrillation, a current depolarizes acritical mass of myocardial cells so that the remaining myocardial cellsare not sufficient to sustain the fibrillation. The electric shock maythus terminate the tachyarrhythmia by depolarizing excitable myocardium,which thereby prolongs refractoriness, interrupts reentrant circuits,and discharges excitatory foci.

Implantable cardioverter/defibrillators (ICDs) provide electro-therapyby delivering a shock pulse to the heart when fibrillation is detectedby the device. The ICD is a computerized device containing a pulsegenerator that is usually implanted into the chest or abdominal wall.Electrodes connected by leads to the ICD are placed on the heart, orpassed transvenously into the heart, to sense cardiac activity and toconduct the impulses from the pulse generator. Typically, the leads haveelectrically conductive coils along their length that act as electrodes.ICDs can be designed to treat either atrial or ventriculartachyarrhythmias, or both, by delivering a shock pulse that impresses anelectric field between the electrodes to which the pulse generatorterminals are connected. The electric field vector applied to the heartis determined by the magnitude of the voltage pulse and the physicalarrangement of the shocking electrodes, which may serve to concentratethe field in a particular region of the heart. Thus, the particularelectrode arrangement used will dictate how much depolarizing current isnecessary in order to terminate a given tachyarrhythmia.

Ventricular and atrial fibrillation are phenomena that exhibit athreshold with respect to the shock magnitude and duration needed toterminate the fibrillation by changing the transmembrane potential in acritical mass of myocardial cells. The ventricular defibrillationthreshold (VDFT), for example, is the smallest amount of energy that canbe delivered to the heart to reliably convert ventricular fibrillationto normal sinus rhythm. Similarly, the atrial defibrillation threshold(ADFT) is the threshold amount of energy that will terminate an atrialfibrillation. The larger the magnitude of the shocks delivered by anICD, the more the battery is drained, thus decreasing the longevity ofthe device. It is desirable, therefore, for the defibrillation thresholdto be as small as possible in order to minimize the amount of shockingcurrent that the ICD must deliver in order to terminate a giventachyarrhythmia.

Electrode arrangements have been devised in an attempt to minimize thedefibrillation threshold for particular types of tachyarrhythmias. Forexample, the traditional configuration for ventricular defibrillation isto place a cathodic electrode in the right ventricle, with the anodeformed jointly by an electrode placed in the superior vena cava and theconductive housing of the ICD acting as an additional electrode. Fortreating atrial fibrillation, a conventional electrode configuration isto use electrodes disposed within the coronary sinus and in the rightatrium. In addition, the waveform of the shocking pulse also affects thedefibrillation threshold. ICDs use a capacitor discharge system fordelivering shock pulses in which a charged capacitor is connected to theshock electrodes to deliver current to the myocardium. Because of spaceconstraints, the size of a typical capacitor is limited and thusexhibits a significant exponential decay when connected to the load(i.e., a small RC time constant). Rather than allowing the decay tocontinue when the capacitor is connected across the load, solid-stateswitches may be used to sharply truncate the waveform which may resultin a lower energy requirement for defibrillation. ICDs also commonlyemploy a biphasic shock pulse waveform in which the polarity of thewaveform reverses during the shock pulse, a technique that has beenfound to further lower the defibrillation threshold. (See U.S. Pat. No.4,998,531, hereby incorporated by reference.)

In order to further improve safety and avoid unnecessary discomfort forICD patients, there is a continuing need for methods and apparatus thatimprove the efficiency of electrical defibrillation and thereby reducethe defibrillation threshold. Such reductions in defibrillationthresholds may also expand the population of patients for whom ICDs arean appropriate therapeutic option. It is toward this general objectivethat the present invention is directed.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for terminatingtachyarrhythmias such as fibrillation by the efficient delivery ofelectrical energy through an electrode configuration to the heart. Ashock pulse output circuit in accordance with the invention includes astorage capacitor with positive and negative terminals and furtherincludes at least three electrode lead terminals with each suchelectrode lead terminal switchably connected to the positive andnegative capacitor terminals. Control circuitry then switches selectedelectrode lead terminals to either the positive or negative capacitorterminal in order to impose the capacitor voltage between electrode leadterminals and deliver a shock pulse. By serially switching selectedelectrode lead terminals to selected capacitor terminals, the controlcircuitry may thus generate a defined shock pulse waveform. For example,selected electrode lead terminals can be switched between differentcapacitor terminals in a manner that reverses the polarity of thevoltage between the terminals to deliver a biphasic or multiphasic shockpulse. The particular electrodes used to deliver a conventional shockpulse are determined by the programming of the control circuitry,allowing for easy modification. The multi-terminal pulse output circuitalso allows for more complex shock pulse waveforms to be generated byusing different electrode lead terminals during the same pulse outputcycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of an apparatus for terminatingtachyarrhythmias with electrical energy.

FIG. 2 shows the pulse output circuitry in an exemplary embodiment ofthe invention.

DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 1 is a system diagram of a microprocessor-based implantablecardioverter/defibrillator with the capability of also delivering pacingtherapy. A microprocessor 10 communicates with a memory 12 via abidirectional bus. The memory 12 typically comprises a ROM or RAM forprogram storage and a RAM for data storage. The ICD has atrial sensingand pacing channels comprising electrode 34, lead 33, sensing amplifier31, pulse generator 32, and an atrial channel interface 30 whichcommunicates bidirectionally with a port of microprocessor 10. Theventricular sensing and pacing channels similarly comprise electrode 24,lead 23, sensing amplifier 21, pulse generator 22, and a ventricularchannel interface 20 which communicates bidirectionally with a port ofmicroprocessor 10. For each channel, the same lead and electrode areused for both sensing and pacing. The sensing channels are used tocontrol pacing and for measuring heart rate in order to detecttachyarrhythmias such as fibrillation. The ICD detects a ventriculartachyarrhythmia, for example, by measuring a heart rate via theventricular sensing channel and determining whether the rate exceeds aselected threshold value. A telemetry interface is also typicallyprovided enabling the programming of the microprocessor to be modifiedusing an external programmer.

A pulse output circuit 50 is also interfaced to the microprocessor fordelivering cardioversion or defibrillation pulses to the heart viaelectrode lead terminals 51 a through 51 c that are connected byelectrode leads to shock electrodes 52 a through 52 c placed inproximity to regions of the heart. The electrode leads have along theirlength electrically conductive coils that act as electrodes fordefibrillation stimuli. The electrode leads and electrodes used in anyof the described embodiments below may be implemented as lead-bodyelectrodes that are either single elongated coils or made up of aplurality of smaller bands. The pulse output circuitry as well as therest of the device are powered by a battery power supply. The device isenclosed by a case which may be implanted by placing it an abdominalwall pocket, or preferably, in a pectoral pocket either subcutaneouslyor under the pectoralis major muscle. The leads from the housing aretypically advanced to the heart transvenously, with venous accessthrough the cephalic or subclavian veins.

FIG. 2 shows a pulse output circuit in accordance with the presentinvention. An energy storage capacitor C1 is used to store an electricalcharge which is delivered to the heart in the form of a selected pulsewaveform. The capacitor C1 may be a single capacitor or may be made upof multiple capacitors connected in series or parallel. In any case, thecapacitor C1 is charged to a high voltage from a low voltage battery bya boost converter BC that is basically a step-up switching voltageregulator. The shock electrodes are connected to electrode leadterminals 51 a through 51 c which are switchably connected to the energystorage capacitor C1 by switches S1 through S6. When a shock pulse isdelivered, selected electrode lead terminals are connected by theaforementioned switches to the positive or negative terminals of thecapacitor C1 to thereby impress the capacitor voltage across the shockelectrodes to which the selected terminals are connected. Switches S1,S2, and S3 are remote gate thyristors or silicon controlled rectifiershaving gate voltages that are controlled by the microprocessor 10.Switches S4 through S6 are insulated gate bipolar transistors alsohaving microprocessor-controlled gate voltages. By controlling the stateof the switches, the microprocessor can control the polarity of theshock pulse delivered to selected electrodes to deliver monophasic,biphasic, or multiphasic shock waveforms. As will be described below,the microprocessor may also select different electrodes during the pulseoutput cycle.

The pulse output circuit described above permits a great deal offlexibility with respect to the type of shock pulse waveform that may bedelivered. For example, a selected pair of electrode lead terminals maysimply be switched to different terminals of the energy storagecapacitor and then switched off to deliver a monophasic shock pulse. Byswitching the lead terminals to opposite capacitor terminals midwaythrough the pulse cycle in order to reverse the polarity of thewaveform, a biphasic pulse may be delivered. Similarly, the polarity ofthe waveform can be reversed multiple times to deliver a multiphasicpulse. These types of shock waveforms can be generated by a two-terminalH-bridge type of output circuit. As will be described below, however, amulti-terminal output circuit provides additional pulse outputconfigurations such as using different pairs of electrodes to deliversuccessive shock pulses, connecting certain electrodes in common duringthe pulse, and generating more complex waveforms using multipleelectrodes that can vary spatially as well as temporally during thepulse cycle.

In one embodiment, a pulse output configuration can be implemented whichis particularly suited for terminating atrial arrhythmias. In thisconfiguration, an electrode lead is connected to a first shockingelectrode which is situated in the coronary sinus such that theelectrode resides in the left lateral heart. The electrode lead terminalis then switched to a capacitor terminal so as to act as a cathodeduring a monophasic voltage pulse. A second shocking electrode isdisposed within the superior vena cava and is switched through its leadto the other capacitor terminal so as to form an anode during thevoltage pulse. The case of the device may be switched to a capacitorterminal so as to form a third electrode in common with one of theothers. For example, the coronary sinus electrode may act as the solecathode while the superior vena cava and case electrodes act as jointanodes for a monophasic defibrillation stimulus. In other embodiments,the polarity of the waveform can be reversed or can be reversedmid-cycle to generate a biphasic waveform. In another embodiment, apulse output configuration that is particularly suited for ventriculardefibrillation is employed. A lead with a first distal shockingelectrode is situated in the right ventricle, with its electrode leadswitched to one terminal of the capacitor so as to act as a cathodeduring a monophasic voltage pulse. Second and third shocking electrodesare switched through their respective leads to the other terminal of thecapacitor so as to form a joint anode during the voltage pulse and aredisposed within the superior vena cava and coronary sinus, respectively.The conductive case of the device may be switched to a capacitorterminal in common with the second and third electrodes so as to alsoconstitute the joint anode. Many other pulse output configurations usingarrangements of multiple shocking electrodes are, of course, possible.For example, a four electrode arrangement may be used which includes thedevice case as one electrode and shocking electrodes disposed in thesuperior vena cava, the coronary sinus, and the right ventricle.

Although a conventional two-terminal pulse output circuit may havemultiple electrodes hardwired to its capacitor terminals in order toimplement either of the just described pulse output configurations, amulti-terminal pulse output circuit in accordance with the presentinvention may be programmed to implement any pulse output configurationthat its physical arrangement of electrodes is capable of supporting.Using an external programmer, the pulse output algorithms executed bythe microprocessor may be modified so as to use any of the connectedelectrodes as cathodes or anodes during a pulse output. Thus, aclinician may select a particular pulse output configuration initiallyand change to another one after an evaluation period by simplyreprogramming the device.

The pulse output configurations described so far have been ones in whichselected shock electrodes are grouped together throughout the pulseoutput cycle while being switched to one or both capacitor terminals. Apulse output circuit in accordance with the present invention, however,may be programmed to serially switch any of the physically connectedshock electrodes to either of the capacitor terminals at selected timesduring the pulse output cycle. Complex pulse output waveforms, in whichdifferent electrodes are used as cathodes and anodes during the pulseoutput cycle, may thus be delivered.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. An apparatus for delivering defibrillation shock pulses, comprising:pulse output circuitry for delivering shock pulses, the circuitryincluding a storage capacitor with positive and negative terminals andfurther including at least three electrode lead terminals with each suchelectrode lead terminal switchably connected to the positive andnegative capacitor terminals; control circuitry for switching a selectedelectrode lead terminal to either the positive or negative capacitorterminal to impose a capacitor voltage between electrode lead terminalsand deliver a shock pulse, and, wherein the control circuitry isconfigured to output a shock pulse by serially switching a plurality ofselected pairs of electrode lead terminals to selected capacitorterminals and thereby deliver a multiphasic shock pulse which variesboth temporally and spatially.
 2. The apparatus of claim 1 furthercomprising shock electrodes for connection to selected electrode leadterminals and disposition in proximity to the heart.
 3. The apparatus ofclaim 2 further comprising a device housing and wherein at least one ofthe electrode lead terminals is connected to the device housing.
 4. Theapparatus of claim 2 wherein the shock electrodes include a firstelectrode adapted for disposition within the coronary sinus, a secondelectrode adapted for disposition within the superior vena cava or rightatrium, and a third electrode.
 5. The apparatus of claim 4 wherein thethird electrode is adapted for disposition in the right ventricle. 6.The apparatus of claim 4 wherein the third electrode is an implantablehousing.
 7. The apparatus of claim 1 further comprising: a sensingchannel for detecting electrical events in the heart and producingsensing signals in accordance therewith; processing circuitry fordetecting the occurrence of a tachyarrhythmia from the sensing signals;and, wherein the control circuitry is configured to deliver amultiphasic shock pulse upon detection of a tachyarrhythmia.
 8. Theapparatus of claim 7 wherein the sensing channel is a ventricularsensing channel and the processing circuitry is configured to detect theoccurrence of ventricular tachyarrhythmias by measuring a heart rate viathe ventricular sensing channel.
 9. The apparatus of claim 7 wherein thesensing channel is an atrial sensing channel and the processingcircuitry is configured to detect the occurrence of atrialtachyarrhythmias by measuring a heart rate via the atrial sensingchannel.
 10. The apparatus of claim 1 further comprisingvoltage-controlled metal-oxide semiconductor switches for switching theelectrode lead terminals to a capacitor terminal.
 11. The apparatus ofclaim 1 further comprising remote gate thyristors for switching theelectrode lead terminals to a capacitor terminal.
 12. The apparatus ofclaim 1 further comprising silicon controlled rectifiers for switchingthe electrode lead terminals to a capacitor terminal.
 13. The apparatusof claim 1 further comprising insulated gate bipolar transistors forswitching the electrode lead terminals to a capacitor terminal.
 14. Theapparatus of claim 1 wherein the storage capacitor is a singlecapacitor.
 15. The apparatus of claim 1 wherein the storage capacitor ismade up of a plurality of capacitors connected in series or parallel.16. The apparatus of claim 1 wherein the pulse output circuit includes abattery and a boost converter for charging the storage capacitor. 17.The apparatus of claim 1 wherein the electrode lead terminals areconnected to a four electrode arrangement which includes the device caseas one electrode and shocking electrodes disposed in the superior venacava, the coronary sinus, and the right ventricle.
 18. The apparatus ofclaim 1 further comprising a telemetry interface by which a particularpulse output configuration used by the apparatus may be modified.